SPC AC 170549

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1249 AM CDT Mon Jul 17 2017

   Valid 171200Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   NORTHERN PLAINS AND UPPER MS VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL PA TO
   VT...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   NORTHERN HIGH PLAINS TO WESTERN LAKE SUPERIOR...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM NORTHERN VA
   TO NH...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF AZ...

   ...SUMMARY...
   Isolated thunderstorms are expected this afternoon and evening from
   South Dakota into northern Minnesota, and during the day from
   Pennsylvania into New York and Vermont. Isolated severe storms are
   expected from Wyoming into Nebraska, across the Appalachians, and
   across southern Arizona.

   ...Northern Plains/Upper MS Valley...

   Strong mid-level short-wave trough is forecast to progress across
   AB/SK into MB by 18/00z. This feature will flatten upper ridge over
   the northern Plains/upper MS Valley as weak height falls spread
   along the international border. Associated surface front should
   surge across much of ND by early afternoon with a pre-convective
   frontal position expected to extend from near INL, southwest into
   central SD at peak heating. Latest model guidance suggests strong
   boundary-layer heating will result in surface temperatures soaring
   through the upper 90s to near 100F across SD into extreme western MN
   which should remove CIN as convective temperatures are breached.
   While large-scale forcing for ascent should primarily spread across
   southern Canada, frontal forcing and strong surface heating should
   result in isolated thunderstorm development after 21z from northwest
   MN into central SD. Forecast soundings suggest initial activity may
   be high based within veered southwesterly low-level flow. However,
   modest mid-high level flow should allow this activity to mature
   downstream across MN where buoyancy is forecast to be considerably
   higher due to more moist boundary layer. A few supercells could
   evolve across northern portions of the SLGT risk where surface-6km
   bulk shear is forecast to be on the order of 35kt. Damaging
   winds/large hail are the primary severe threats with this activity.

   ...PA to Southern VT...

   Early-morning water vapor imagery depicts a pronounced mid-level
   trough digging east-southeast across the Great Lakes/OH Valley
   region. Latest model guidance suggests this feature will extend
   across western NY/PA at 18z with modest southwesterly high-level
   flow expected to extend downstream into southern New England at peak
   heating. Relatively cool 500mb temperatures (-10 to -12C) will
   overspread much of the northeastern US during the afternoon which
   should aid destabilization as surface temperatures warm. Forecast
   soundings suggest thunderstorms that develop across this region
   could be robust with isolated damaging winds and marginally severe
   hail being the primary severe threats.

   ...AZ...

   Subtle day-to-day changes will continue across the southwestern US
   early this week as center of upper ridge holds near the Four-corners
   region. Convection will once again be modulated by daytime heating
   and orographic influences as PW values remain seasonally high with
   little notable drying. For these reasons, scattered strong
   thunderstorms should develop across the higher terrain during the
   late afternoon then propagate toward lower elevations where gusty
   winds will be possible, especially if convection organizes due to
   adequate cold-pool development.

   ..Darrow/Gleason.. 07/17/2017

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